Ethylene oxide is produced commercially by the silver-catalyzed partial oxidation of ethylene with oxygen. The oxygen source may be commercially available oxygen or air. Generally, in an oxygen-based process, ethylene, oxygen and a ballast gas are mixed with a recycle gas and fed to the reactor. The reactor comprises a number of tubes which are placed inside a vessel and arranged similar to shell and tube heat exchangers. The reactor tubes are filled with a silver catalyst placed on a porous support containing small amounts of promoters. A coolant circulates in the shell around the reactor tubes to maintain temperature control.
In an oxygen-based process, a typical composition of the gas stream fed to the reactor tubes includes 20 to 30 mol % ethylene, 5 to 10 mol % oxygen, 4 to 20 mol % argon, 30 to 50 mol % ballast gas, 1 to 15 mol % carbon dioxide, with ethane, water and a small amount of ethylene dichloride constituting the remainder of the composition. Ethylene reacts with oxygen to form the reaction product ethylene oxide, and byproducts carbon dioxide and water inside the catalyst filled tubes. The reactions are exothermic and hot spots may be formed within the reactor tubes. "Hot spots" are localized areas of high temperature in the reactor. Hot spots may cause an unwanted runaway reactive condition if not properly controlled.
In an oxygen-based process, the ballast gas is introduced into the reactor in order to obtain optimum reaction mixtures, to control temperature and to avoid the presence of flammable mixtures. High temperature favors the undesirable production of carbon dioxide and reduces both catalyst activity and catalyst life. By controlling the reaction rates and the removal of heat from the reaction zone, the temperature and location of the hot spots can be controlled in order to maximize selectivity to ethylene oxide and preserve the catalyst activity. The ballast gas systems disclosed in the past have included methane, ethane, nitrogen, carbon dioxide, and mixtures of these gases. Methane is preferable to nitrogen as a ballast gas because it has better thermal properties. These thermal properties include a higher molar heat capacity and higher thermal conductivity. U.S. Pat. No. 3,119,837 discloses that ethylene oxide yield will increase as methane replaces nitrogen in the ballast gas.
In an oxygen-based process, the reactor effluent is treated in two separate steps: first, remove product ethylene oxide; and second, remove byproduct carbon dioxide. The remaining gas is recycled to the reactor after a portion of it is purged. A significant amount of ethylene is lost to the purge stream as a selectivity loss. Ballast gas is also lost in the purge stream and must be replenished with fresh ballast gas feed. The purge is required in order to keep impurities in the gas mixture entering the reactor at acceptable levels. One impurity, argon, is introduced in the oxygen stream. Other impurities, like ethane or propane, are found in the ethylene feed stream.
Ethylene oxide production may involve an air-based process instead of an oxygen-based process. In the air-based process, unreacted gases may be recycled to the reactor, but the extent of this recycling is limited by the necessity to remove excess nitrogen from the process. Nitrogen is continuously added to the process as air is added to the oxidation reactor. When nitrogen is removed, an appreciable portion of the unreacted ethylene is lost along with the nitrogen. In order to limit the loss of ethylene under these conditions, the withdrawn gases are mixed with additional air and passed through one or more additional oxidation reactors in the presence of the silver catalyst under more extreme reaction conditions. However, the additional oxidation reactor (or reactors) significantly increases the capital cost of the ethylene oxide plant. Another disadvantage of an air-based process is that a lower rate of ethylene conversion is obtained.
Major differences exist between the purging of air-based and oxygen-based processes. The air-based process requires a substantial purge stream and a staged reaction-absorption system. With the oxygen-based stream, there is a significant reduction in the amount of inert gases introduced into the closed cycle as compared to the air-based process, resulting in a substantially smaller purge and an almost complete recycle of the unconverted ethylene. However, the carbon dioxide formed in the reactor must be removed on a continuous basis. Additionally, a process purge is required in order to prevent the accumulation of argon in the recycle gas. Argon is a major impurity in an oxygen supply derived from a cryogenic air separation plant. The oxygen source for ethylene oxide production plants is typically a cryogenic plant.
A typical oxygen purity usage in ethylene oxide production plants using the oxygen-based process is from 95% to 99.5%. Various methods for treating the purge from oxygen-based ethylene oxide production plants to recover ethylene have been proposed. For example, U.S. Pat. No. 4,904,807 discloses the use of an argon selective membrane that is used to treat the purge and separate it into two streams 1) an argon rich stream that is vented and 2) an ethylene rich stream that can be recycled back to the ethylene oxide reactor, and U.S. Pat. No. 4,769,047 discloses the use of pressure swing adsorption to remove ethylene from the purge and recycle it back to the reactor. A major disadvantage of these methods is the large capital cost of the associated equipment.
Other patents have discussed ethylene oxide production. U.S. Pat. No. 3,083,213 discloses that the use of high purity oxygen reduces ethylene oxide yield. U.S. Pat. No. 5,262,551 discloses a process for epoxidation of ethylene, wherein ethylene is reacted with oxygen in a mole ratio of between three to nine, in the presence of a silver metal catalyst and a halide gas inhibitor, at a pressure of about 200 to 300 psig. Ethylene feed contained therein is about 30 to 90 mol % ethylene. The particularly high concentration of ethylene feed has been disclosed to demonstrate improved selectivity.
It is believed that there has not been a commercially practical solution to reduce the impurities associated with the production of ethylene oxide. Therefore, there is a need to provide a new method for producing ethylene oxide which maximizes the selectivity and minimizes ethylene loses to purge, thus improving the yield of the ethylene oxide production process.